How to handle N to 1 entity relationships

Users Dgraph
,
4

I was looking at this yesterday a bit and thinking I could maybe simplify it to something like this:

	q := `
	{
		node(func: eq(xid, "` + uid.Value() + `")) {
			u as uid
		}
	}
	`

	node := map[string]string{"uid": "uid(u)"}

	// Only delete Resource if it has no inbound edges
	cond := `@if(not type(Resource) OR eq(count(~resource), 0))`

	pb, err := json.Marshal(node)
	if err != nil {
		return nil, fmt.Errorf("json marshal: %w", err)
	}

	mu := &dgapi.Mutation{
		Cond:    cond,
		SetJson: pb,
	}

	req := &dgapi.Request{
		Query:     query,
		Mutations: []*dgapi.Mutation{mu},
		CommitNow: true,
	}

	_, err = s.c.NewTxn().Do(context.Background(), req)
	if err != nil {
		return fmt.Errorf("txn Delete: %w", err)
	}

But when I try to do this I get the following error:

2021/02/20 11:08:55 txn Delete: rpc error: code = Unknown desc = Some variables are defined but not used
Defined:[__dgraph__0 u]
Used:[__dgraph__0]

This, I assume, means that u does not seem to be used, however, it actually is?

	node := map[string]string{"uid": "uid(u)"}

	pb, err := json.Marshal(node)
	if err != nil {
		return nil, fmt.Errorf("json marshal: %w", err)
	}

I’m not quite sure what the problem is here, given, for example, the following works like a charm:

	query := `
	{
		resource(func: eq(xid, "` + uid.Value() + `")) {
			u as uid
	        }
	}
	`

	res := &Resource{
		UID:        "uid(u)",
	}

	pb, err := json.Marshal(res)
	if err != nil {
		return nil, fmt.Errorf("json marshal: %w", err)
	}

	mu := &dgapi.Mutation{
		Cond:    cond,
		SetJson: pb,
	}

	req := &dgapi.Request{
		Query:     query,
		Mutations: []*dgapi.Mutation{mu},
		CommitNow: true,
	}

	if _, err := s.c.NewTxn().Do(ctx, req); err != nil {
		return fmt.Errorf("txn Add: %w", err)
	}

This puzzles me a bit, but I’m sure I’m missing something fundamental here.